Method for determining minor quantity of strong acid in major quantity of weak acid



Sept. 9, 1969 E. J. TREBES 3,466,228

METHOD FOR DETERMINING MINOR QUANTITY OF STRONG ACID IN MAJOR QUANTITY OF WEAK ACID Flled Feb. 28, 1967 s Sheets-Sheet 1 REACTIOIBFPRODUCT F I G I I STRONG ACID ACIDULATION MIXTURE CONTAINING NON AQUEOUS SOLVENT QUANT'TY OF SECOND STRONG ACID ACIDULATION PRODUCT OPTIONS a REACTANT DILUTION (OPTIONAL) MIX IN G A ELECTRO-CHEMlCAL pH g' MEASURING RECORDING STRONG ACID EXCESS (H2504 40 I I I PRIMARY IONIZATION STAGE SECONDARY IONIZATION STAGE TERTIARY IONIZATION STAGE STRONG Acm DEMAND INVENTOR F I e .2

ATTORNEY Sept. 9, 1969 E- J. TREBES 3,466,228

METHOD FOR DETERMINING MINOR QUANTITY OF STRONG ACID IN MAJOR QUANTITY OF WEAK ACID Filed Feb. 28, 1967 3 Sheets-Sheet 2 DlSCHARGE DISCHARGE INVENTOR ATTORNEY Sept. 9, 1969 E. J. TREBES 3,466,228

METHOD FOR DETERMINING MINOR QUANTITY OF STRONG ACID IN MAJOR QUANTITY OF WEAK ACID Filed Feb. 28, 1967 s Sheets-Sheet s FIG.5

FIG.4

INVENTOR EDWARD J. mseas ATTORNEY United States Patent 3,466,228 METHOD FOR DETERMINING MINOR QUANTITY OF STRONG ACID IN MAJOR QUANTITY OF WEAK ACID Edward J. Trehes, Tampa, Fla., assignor to Tennessee Corporation, New York, N.Y., a corporation of Delaware Filed Feb. 28, 1967, Ser. No. 619,237 Int. Cl. 801k 3/00 U.S. Cl. 204-1 9 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for the electrochemical determination of sulfuric acid excess or demand iri'the acidulation of phosphate rock to obtain phosphoric acid product. A small sample of the reaction product is "mixed with an organic solvent such as a mixture of ethylene glycol and isopropyl alcohol in order to substantially reduce the phosphoric acid hydrogen ion dissociation as indicated by conventional pH meter measurements. Additionally, a small and fixed quantity of a strong acid such as hydrochloric acid (HCl) is also mixed in the organic solution of the product sample, to provide hydrogen ion for anv unreacted phosphate rock. The test mixture is then passed through a quinhydrone electrode apparatus and the electro chemical properties of the mixture is measured by conventional pH indicating equipment. The measured pH values are compared to the values obtained during calibration of the equipment utilizing known solutions. The pH measuring equipment thereby provides a direct value for the quantity of strong acid present or required by the reaction product.

BACKGROUND OF THE INVENTION This invention relates to a process and apparatus for determining the quantity of strong acid in a major quantity of a weak acid. More particularly, the invention is directed to a method and means for automatically determining the concentration of H-ion dissociation of a strong acid in a medium containing a large quantity of weak acid product.

Numerous processes in the chemical industry are dependent on chemical reactions conducted in an acidic or basic media Where it is essential to control the concentration of a minor ingredient. An example of such a process is the control of sulfuric acid concentration in the manufacture of phosphoric acid by the wet process. The oldest and still the most economical method of producing crude phosphoric acid is to treat phosphate rock with sulfuric acid, thereby precipitating calcium sulfate and releasing phosphoric acid. Producers of wet process phosphoric acid try to proportion the phosphate rock and sulfuric acid so that there is neither a surplus of acid nor of unreacted phosphate rock. Various schemes for estimating the amount of acids required for acidulating the phosphate rock have been proposed. One method is to calculate the sulfuric acid that will be required to change the oxides of calcium, aluminum, iron, magnesium, and other basic reacting elements of the phosphate rock into sulfates. Since the basic elements in phosphate rock are not present as oxides but as phosphates, sulfates, and fiourides, less acid will be required than would be the case if all the baseforming elements were present as oxides. Accordingly, the correction factor is usually applied dependent upon the specific phosphate rock used, and this of course leads to problems resulting from the lack of uniformity in the phosphate rock raw material.

Numerous prior solutions to the problem have been tried, however none were entirely satisfactory. One method used was the so-called ratio test, which consisted of titrating a sample of acid with caustic (or a base with acid) to two separate end points. The ratio of the two titrations represents a measure of a strong acid or base concentration. This method is rapid, but in industrial practice it is inaccurate due to the presence of impurities such as fluosilicic acid. Other methods have been developed to precipitate an insoluble salt of a strong acid or base such as by the addition of barium chloride to precipitate the sulfate in phosphoric acid. Such a method is described in US. Patent No. 3,158,163 to Claudy, wherein special apparatus is required to make the determination.

SUMMARY OF THE INVENTION It has been found that mixtures of acids (or bases) having sufficient differences in their respective pK value, when mixed in suitable non-aqueous media containing organic solvents would provide a range of differences in hydrogen ion-dissociation so extensive that precise measurements of small changes in acid, or base concentration as measured electrochemically by commercially available instruments provides specific andreproducible indications, which are empirically correlated to the respective acid or base concentration in the original sample. More particularly it has been found that the differences in H+ ion dissociation as given in pK value between the various ionization stages of the strong and weak acid are magnified in the non-aqueous media, thereby virtually diminishing the effect of the weak acid on the electrochemically determined pH value.

Accordingly, this invention relates to a process and apparatus for determining the excess or deficiency of a strong acid (or base) in the reaction product which is weakly either acidic (or basic). The method comprises adding an organic solvent to the reaction mixture which effectively reduces the electro-conductivity of the weak acid or base, and electrochemically measuring the concentration of dissociated hydrogen ion of the resultant test mixture. Electrochemical measurement as by a pH meter or by conductivity will give readings or signals which are proportionately indicative of the concentration of or demand for the minor quantity of the strong acid or base. Such readings or indications can be used either to manually control the amount of reactants added to the raw material or to automatically control the addition of the strong acid or base as required by the process.

It is therefore an object of this invention to provide a method for determining the concentration of a minor quantity of a strong acid in a major quantity of a weak acid.

It is another object to provide a method of determining the concentration of a minor quantity of a strong base in a major quantity of a weak base.

It is an object of this invention to provide an apparatus for automatically measuring the minor quantity of strong acid or base in the major quantity respectively of either Weak acid or weak base.

Another object of this invention is to provide a method for determining the excess concentration or demand for sulfuric acid in the reaction mixture of wet process phosphoric acid.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the invention will be apparent from the following description, examples and accompanying drawings wherein:

FIGURE 1 is a block diagram of a process according to the invention.

FIGURE 2 is a plot of electrical output v. excess concentration or demand for the strong acid, specifically H FIGURE 3 is a flow diagram of the apparatus used to practice the method of this invention.

FIGURE 4 is an elevation of the apparatus showing instrument housing and components.

FIGURE is a detailed view of the EMF measuring electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For purposes of this invention a strong acid is one which has a pK strength exponent approaching infinity in an aqueous solution, where pK is equal to log K and K is the dissociation constant of the acid.

The method as shown in FIGURE 1 for automatically determining the minor quantity of strong acid in a major quantity of weak acid may be applied to the process for acidulation of phosphate rock to obtain phosphoric acid. Numerous strong acids such as either nitric acid (HNO or sulfuric acid (H 50 may be used to react with the phosphate rock, (Ca (POQ to obtain the acidulation product, phosphoric acid. If there is excess acid to react with all salts and oxides present in the phosphate rock, the reaction product will contain a minor quantity of the strong acid (such as HNO or H 80 and a major quantity of phosphoric acid. The various resulting salts of the strong acid are precipitated out and separated, while those relatively soluble remain in solution. To maintain the aforesaid salts in solution, water in a fixed proportion is added to dilute the reaction product. Organic solvents which have been found to be suitable for use either alone or in combination to reduce the electro-conductivity and H+ ion dissociation of the weak acid in the test mixture are low carbon alcohols, glycols and ketones having from 1 to 5 carbon atoms. Examples of such solvents are methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, isoamyl alcohol, and methyl isobutyl ketone. Additionally, glacial acetic acid, dimethyl formamide, and pyridine may be used as organic solvents in the practice of this invention. However, it is preferred to use a mixture of ethylene glycol and isopropyl alcohol. A quantity of a second strong acid (such as hydrochloric acid) is mixed in the organic solvent in sutficient amount to react with any monocalcium phosphate Ca(H PO The organic solvent mixture is then mixed with the reaction product sample to obtain a test sample which in turn will be in one of the three following conditions:

(1) The sample is in the primary ionization stage where all the phosphate rock is reacted to phosphoric acid and a small quantity of the strong acid remains as excess in the test sample;

(2) All the phosphate rock is reacted to phosphoric acid, and all the strong acid has also been reacted; and

(3) The sample is in the secondary ionization stage where all the strong acid has reacted with the phosphate rock and a minor quantity of dissolved monocalcium phosphate is present and unreacted. As the tertiary ionization stage is essentially constant its eifect is negligible in the non-aqueous solvent.

The diluted mixture in the organic solvent is then subjected to an electrochemical measuring Step, where either a potential is passed across a pair of electrodes or an electromotive potential or force (EMF) is developed across the electrodes.

With reference to the diagram of FIGURE 1, where the strong acid used in the reaction is sulfuric acid H 80 variable stages of ionization are obtained. Where there is an excess of strong acid in the reaction, the ionization stages of the product mixture may be grouped as follows.

Primary stage:

As all the phosphate rock Ca (PO has been reacted to phosphoric acid H PO the EMF obtained electrochemically is indicative of the primary stage only and is proportional to the concentration of H SO only.

Where the reaction product is deficient in the strong acid H 50 the ionization stages will be represented by the secondary and tertiary stages, as no strong acid exists for primary dissociation. The EMF will then be proportional to the quantity of Ca(H PO present to remove H+ in the secondary ionization stage. As the quantity of Ca(H PO is proportional to H demand, the EMF of the cell is indicative of percentage H 50 demand. Additionally, since Ca(H PO is relatively insoluble in the organic solvent a second strong acid such as HCl in a fixed amount may be added to the solvent to react with the Ca(H PO to form soluble phosphoric acid and calcium chloride (CaCl The quantity of HCl added to the solvent is in excess of the amount to react the anticipated maximum quantity of Ca(H PO The variable affecting EMF output of the measiuring device is the quantity of dissociated HCl remaining after reaction with the Ca(H PO The quantity of HCl which is unreacted is negatively proportional to the quantity of Ca(H PO which was present in the test sample. As HCl is a strong acid with a pK exponent strength on the same order as H 80 the quantity of HCl reacted is indicative of H 80 demand. Therefore the quantity of H 50 demand is negatively proportioned to the EMF output.

Various electrochemical systems and electrodes for measuring pH have been found to be sufficiently accurate and reproducible for measurements according to this process. Among them are the conductivity method, the glass electrode, and the quinhydrone electrode. For more detailed descriptions of the aforesaid electrochemical systems and electrodes the reader is referred to the Encyclopedia of Chemical Technology by Kirk and Othmer, vol. 7, pp. 711-726 (1951, Interscience Pub. Inc.).

Apparatus for performing the process of this invention is shown in FIGURES 3 through 5 where the reaction product containing a major quantity of weak acid product and either a minor quantity of excess strong acid or a minor quantity of unreacted phosphate rock is fed to the constant level launder 10. A small stream of product is drawn oif through pipe 12 to an acid water propor' tioner 14 where a specific amount of water generally on a 1:2 ratio by volume is added to the reaction product stream, to dilute it and to insure solubility of the salts. The diluted reaction product stream is fed through pipe 16 and clamp pump 18 to a mixing tank 20 where the diluted reaction product stream is thoroughly agitated by any of conventional means such as a Fisher magnetic stirrer 22. The thoroughly mixed diluted reaction product stream is then fed through pipe 24 to a deaerating cell 26 where air is drawn off in pipes 28 and clamp pump 30 and discharged. The diluted deaerated reaction product stream is passed through pipes 32 to mixing clamp pump 34 where a measured portion is drawn oii via valve 35 and the remainder discharged.

The organic solvent is fed through pipes 36 and clamp pump 38 in a measured stream to the mixing clamp pump 34 where it is mixed with the diluted deaerated reaction product to form a test sample. The test sample is pumped through pipe 40 to a second mixing tank 42 mounted on a second stirring means such as 2. Fisher magnetic stirrer 44 and thoroughly mixed. The test sample is then fed through pipe 46 to a microflow test cell 48 such as the Beckman Microflow Cell No. 14683,

in which a platinized gold indicator electrode 50 and a none-aqueous reference electrode 52 are located. The electrodes 50 and 52 are respectively connected to termi' nals 54 and 56. Electrode terminals 54 and 56 are connected to a conventional and known means 58 for measuring EMF across the electrodes, and determining pH thereby. The test sample stream after passing through the microflow test cell 48 is fed through pipes 60 and discharged.

FIGURE 5 is a more detailed view of the microflow test cell 48 and the electrodes 50 and 52. The non-aqueous reference electrode 52 comprises a large glass filter tube 62 which is vertically supported by a holder 64 attached to a fixed support which is not shown. The glass filter tube 62 contains the non-aqueous electroltye 66 which may consist of 1 molar LiCl in a mixture of ethylene glycol and isopropanol in the ratio of 4:1 by volume. A rubber stopper 68 is used to close the top of the glass filter tube 62 and has a vent 70 and a centrally located hole for supporting the electrode portion 72 of the refer ence electrode 52 with the lower portion of the electrode portion 72 in the electrolyte. A conducting wire 74 is attached to the top of the electrode portion 72 and has another end attached to the terminal 56. The lower open end 76 of the glass filter tube 62 is connected to a tube 78 the lower end of which is connected to the reference electrode diffusion tip 80 inserted into the microflow test cell 48. The platinum gold indicator electrode 50 is similarly inserted in the microflow test cell 48 and connected to electrode terminal 54 by conducting wire 82.

In order to more specifically illustrate the process of this invention the following example is given, but with no intention of being limited thereon.

EXAMPLE 1 A sample of wet process phosphoric acid agitator acid comprising a stream of hot 180 F. acid was removed from about 7 ft. below the agitator acid surface, and filtered through a saran cloth. The organic solvent used was ethylene glycol and isopropyl alcohol in the ratio of 3 to 2 by volume. To 2500 ml. of the non-aqueous solvent, ml. of 0.305 N HCl, and 75 ml. of 5% quinhydrone by volume dissolved in acetone was added. The measuring apparatus was an indicator electrode made of platinum and a reference electrode of calomel with satur ated LiCl electrolyte. The sample and solvent were mixed in the ratio of about 1 part sample to 250 parts solvent. The resulting sample-solvent mixture was fed at the rate 15-20 ml. per minute through the measuring cell. The pH recorders used during the tests were Sargent models SR and MR with a Sargent pH adapter between the electrodes and recorder. Utilizing the titration value of the sample to set the recorder pen position, the apparatus was used for approximately 3 to 4 hours before restandardization was made. Over a period of five days the results obtained from the automatic determination of negative strong acid quantity for 60 readings were within 0.17% of the values obtained by titrating samples.

EXAMPLE 2 A continuous stream of hot clarified agitator acid was fed to the proportioner and mixed at the proportioner with 2 parts of untreated water per part agitator acid. A portion of the diluted acid at the rate of 0.18 mls. per minute was mixed with the non-aqueous solvent stream flowing at the rate of 15 ml. per minute to obtain an acid to solvent ratio of 1:250. The non-aqueous solvent, comprises ethylene glycol, and isopropanol in the ratio of 4:1 by volume. Aqueous 4 molar HCl was added to make the solvent 0.002. molar with respect to HCl, and quinhydrone reagent is added at the rate of 0.2% w./v. of the solvent. The acid and solvent mixture was stirred magnetically and passed through the electrode flow cell and a Beckrnan Zeromatic 11 pH Meter, where the output is fed to a recorder. The values obtained automatically were within 2% of that obtained by a manual titration method.

I claim:

1. In a process for determining a minor quantity of a strong acid in an aqueous mixture containing a major quantity of a weak acid, and wherein the aqueous mixture has a total electro-chemical activity substantially due to ionic dissociation of the strong acid and weak acid, the steps comprising:

(a) providing a measured amount of the mixture;

(b) adding a measured effective amount of organic solvent which is capable of suppressing ionic dissociation of the weak acid to the measured amount of the mixture; and

(c) measuring the electro-chemical activity by electrochemical means of the mixture resulting from (b) whereby the measurement is directly proportional to the amount of strong acid present in the mixture.

2. The process of claim 1 wherein a second strong acid is added to the organic solvent in fixed and known amount suflicient to acidulate any unreacted salts in the mixture thereby providing a known electro-chemical activity which is negatively proportional to the amount of strong acid required to acidulate the unreacted salts in the mixture.

3. The process of claim 2 wherein the strong acid is sulfuric acid, the weak acid is phosphoric acid, the organic solvent is a mixture of ethylene glycol and isopropanol; and the second strong acid is hydrochloric acid, which additionally comprises the steps of:

(0) passing the mixture across a pair of electrodes to develop an EMF; and (d) measuring the EMF across the electrodes,

whereby the EMF is directly proportional to the amount of sulfuric acid in the mixture and negatively proportional to the amount of the unreacted salts in the mixture.

4. The process of claim 1 wherein the organic solvent is selected from a group of solvents consisting of alcohols, glycols, and ketones having from 1 to 5 carbon atoms.

5. The process of claim 4 wherein the organic solvent is a mixture of ethylene glycol and isopropanol.

6. The process of claim 1 wherein the electro-chemical activity of the mixture which is measured is electro-conductivity.

7. The process of claim 1 wherein the electro-chemical activity of the mixture which is measured is the EMF.

8. The process of claim 1 wherein the strong acid is sulfuric acid and the weak acid is phosphoric acid.

9. The process of claim 1 wherein the organic solvent is selected from the group consisting of glacial acetic acid, dimethyl formamide and pyridine.

References Cited UNITED STATES PATENTS 1,915,126 6/1933 Handforth 23-230 2,607,718 8/1952 Suthard 204- XR 2,752,307 6/1956 Baran et al. 204-195 2,782,151 2/1957 Suthard 204-195 XR 3,104,946 9/1963 Veal 23-230 XR 3,178,263 4/1965 Karasek et al. 23-165 3,216,925 11/1965 Fanning et al. 208-273 3,262,051 7/1966 Payne 204-1 XR 3,294,652 12/1966 Banks et al. 204-1 JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner US. Cl. X.R. 

